40K, with Warp technology, makes it pretty easy to explain away most pairings of hostile armies.
Warhammer Fantasy though, with the slow speed at which armies travel and there spacing across the known world - at makes explaining why 2 armies are fighting more difficult. Empire, as an example, at war with Vampire Counts, Orcs, Chaos, Bretonnia, or other portions of the Empire all make perfect sense. Empire at war with anyone else requires a great journey.
So tonight my Marienburg Empire force is doing battle with Chad's Ogre Kingdoms. We're separated by most of a continent and some of the most dangerous lands in the Warhammer World. He didn't envision his Ogres as mercenaries, which would have been the easy explanation, so instead my newly dubbed Marienburg Merchant Guard are attempting an overland journey to Ind. I detailed the jouney and gave Chad the option of choosing the point of attack that worked best with how he envisioned his army, and the start of the narrative was established. Here are the initial bits of narrative we each wrote:
"Here is your itinerary for your journey to Ind. You will set sail from Marienburg along the River Reik as soon as the winter breaks. You will head eastward until you reach the mouth of the Talabec - this will take you to Kislev. Be ever vigilant once you pass the mouth of the Urskoy, dark creatures lurk everywhere that far north!
At Kislev you may give your men 3 days rest, the hardest part of the journey will be ahead. You will continue north east along the Upper Talabec until you can travel no further. You will then follow the track to Peak Pass - this will be your path across the World's Edge Mountains. I would suggest acquiring a guide from either Kislev or Ostermark to assist you in your travels into the mountains.
The journey across the mountains will be arduous, but your travels will get more dangerous still when you enter the Dark Lands. It goes without saying, but once you leave the confines of the Empire you should only travel by day and each night keep your light minimal so as not to attract the wicked and barbarous.
Continue to follow the track through the Dark Lands due east. You will come to a river. Rumor tells of a shack and a hermit who sells boats but these cannot be validated nor do they seem likely. I will be providing you with the necessary tools and a couple of craftsmen to help you cobble some barges to take you down this river. Follow it to its mouth and you will reach the exotic and civilized land of Ind.
You will be transporting some of our finest goods. Protect them with your life, trade them judiciously, and then return to Marienburg. If you succeed you will be rich beyond measure and your men and their families will live comfortably for many years.
There is much to prepare for, winter will break within the month and you will need men and supplies. Inside of 6 months we will all be much wealthier - Sigmar be with you.
-Fredrik von Hedlerberg"
Gregor Mountaineater of the Iron Storm tribe sat and brooded, idly picking the remains of a Gnoblar from his teeth. Climbing Mount Skabrand was no simple feat, yet what was his reward? To sit here, waiting to meet a trade caravan that Tyrant Nazlar had agreed to escort through the Vale of Woe. No fighting, no looting, no meat fresh from a clubbing... This was no life for an Ogre.
Gregor's brother, Denor Maw Bitten, approached. "The man-things approach. Better to be eating them than watching them, damn Nazlar's teeth. Me thinks he's lost his teeth and his gut, been eating too much gold."
"Aye, was just thinking the same," replied Gregor. "But what can we do? He's the Tyrant, we's the followers. We've got our little band of bashers here, he's got the rest."
"But we've the Maw on our side," growled Denor. There was blood on his face, and a strange light in his eyes. Gregor sighed inwardly. "You's been snacking on Chaos Dwarves again Denor? They's no good for you, and you know it."
Denor roared, shaking a blood covered cleaver to the sky and casually decapitating a nearby Gnoblar. "The Maw speaks, and I listen! Eat, the Maw says! Gorge, kill, crush, eat! Meat, that's what we want, meat and glory for the Iron Storm, not gold, not this trade Nazlar makes. The Maw says we kill him, eat him, and then..."
As his brother trailed off, Gregor considered. Nazlar had been looking a little weak and gaunt lately, his gut plate getting a little loose. And anyway, a nice little pit fight and snack after would be more interesting than following orders all day. "All rights, you talk sense. But what then? and what now? How's do we start this?"
Denor grinned, showing his blood and marrow stained teeth. "After Nazlar, we takes the tribe and we obey the Maw. We kill, we eat, and we do it again. And to start..." Denor pointed the cleaver at an approaching caravan, barely visible in the distance. "We feast on the weak ones, the man things, in the name of the Maw."
Ogre text by Chad McMann
The only wrench in the plan is that my 1000 point list for tonight includes a Mortar - a weapon not really suited for protecting merchants on the go. Could it be a trade good that we were forced to use out of desperation? This could be. If destroyed in the battle it will be left to rot and rust in the Vale of Woe. If it survives, it will persist with us, even when inconvenient because frankly, we've lugged it all the way through the Empire, across the World's Edge Mountains, and across the Dark Lands - we are not discarding it now!